The most successful decentralized communication system - email as it turned out, people would concentrated to large free provider like Google. Decentralized server does not protect privacy for normal user because not most people could handle owning their server.
The most success decentralized service is BitTorrent. It is decentralized and it is decentralized in client level. Though it also caused uncontrollable piracy, since it is too easy to spread any data using Bittorrent. I think a true decentralized social network to protect privacy should be a p2p app, not server to server federation.
Have you had a look at https://github.com/ssbc/patchwork ? I think it ticks all of the boxes you mentioned!